This invention relates to a semiconductor device, and more particularly, to a nonvolatile semiconductor memory system such as a phase-change memory or a phase-change random access memory (RRAM), and its controller.
There are a phase-change memory and a phase-change optical disk as recording technologies which use physical properties of chalcogenide materials. As phase-change materials used therefor, chalcogenide materials including tellurium (Te) are known.
U.S. Pat. No. 5,254,382 discloses an optical disk medium which uses, for a storage layer, a chalcogenide material represented by [(GeyTe1-y)a(SbzTe1-z)1-a]1-b(In1-xTex)b (in this case, 0.4≦y≦0.6, 0.3≦z≦0.6, 0.4≦z≦0.6, 0.1≦a≦0.5, 0.01≦b≦0.3). This chalcogenide material contains In (indium) added to Ge (germanium)-Sb (antimony)-Te for the purpose of increasing stability of an amorphous state to improve long data-retention while maintaining characteristics which enable fast crystallization.
U.S. Pat. No. 5,883,827 discusses a nonvolatile memory which uses a chalcogenide material film. This nonvolatile memory is a phase-change memory in which stored information is written by a change of atomic arrangement of a phase-change material film made according to Joule heat by a current flowing through the phase-change material film itself and a cooling speed. For example, in the case of setting an amorphous state, a temperature exceeding 600° C. with Joule heat is added to the phase-change material film to melt the phase-change material film. Thus, while an operation current easily becomes large, a resistance value changes by 2 to 3 orders of magnitude depending on a state.
Concerning the electric phase-change memory, studies have been pursued mainly on those using a chalcogenide Ge2Sb2Te5. For example, JP 2002-109797 A discloses a recording element which uses Ge2Sb2Te5. JP 2003-100991 A discloses a technology regarding a memory which uses a chalcogenide material. IEEE International Electron Devices meeting, TECHNICAL DIGEST, 2001, p. 803 to 806 describes rewritability of 1012 times at a phase-change memory which uses a phase-change film containing Ge2Sb2Te5. Nature Materials, Vol. 4, 2005, p. 347 to 351 discloses a technology regarding a phase-change memory which uses a crystal-growth-type material.
JP 2005-050424 A discusses, to deal with characteristic variances among memory elements, a method of performing rewriting with pulses suited to the memory elements by carrying out trial writing for the respective memory elements.
Additionally, there is known a memory element called RRAM in which an electrode of Au (gold), Ag (silver), Cu (copper), Ni (nickel), or Zn (zinc) is formed adjacently to a film of a material containing an oxide, Se (selenium), or S (sulfur). IEEE International Electron Devices meeting, TECHNICAL DIGEST, 2007, P. 771 to 774 describes an application example of a diode to an element for selecting a memory element to reduce a memory cell area. Circuit operations are similar, and hence the diode can be applied to the phase-change memory or a PRAM.
A characteristic operation of a sensor network is an intermittent operation. This operation drives necessary hardware only when a task such as sensing or data transmission is executed, completely stops peripheral hardware when there is no task to be executed, and causes a microcomputer to sleep in a low-power mode. The intermittent operation enables the controller to operate with low power consumption for a long period of time.